Question

I have the following code snippet:

public static void foo(Object x) {
    System.out.println("Obj");
}
public static void foo(String x) {
    System.out.println("Str");
}

If I call foo(null) why is there no ambiguity? Why does the program call foo(String x) instead of foo(Object x)?

Answer 1

why the program calls foo(String x) instead of foo(Object x)

That is because String class extends from Object and hence is more specific to Object. So, compiler decides to invoke that method. Remember, Compiler always chooses the most specific method to invoke. See Section 15.12.5 of JLS

If more than one member method is both accessible and applicable to a
method invocation, it is necessary to choose one to provide the
descriptor for the run-time method dispatch. The Java programming
language uses the rule that the most specific method is chosen.

The informal intuition is that one method is more specific than
another if any invocation handled by the first method could be passed
on to the other one without a compile-time type error.

However, if you have two methods with parameter – String, and Integer, then you would get ambiguity error for null, as compiler cannot decide which one is more specific, as they are non-covariant types.